The invention relates to a colour selection circuit for colour television comprising several inputs and an output, the output carrying a switching signal if signals are present at the inputs in a predetermined combination which corresponds to a given colour.
Such a circuit is disclosed in U.S. Pat. No. 4,051,520. The colour selection circuit, known as a chroma-keying signal generator, does not operate with colour difference signals but rather with chromaticity signals. An advantage is that an improved signal separation is obtained when the keying, or switching, signal is produced when, instead of the luminance dependent colour difference signals, the luminance independent chromaticity signals are used. Due to the fact that generating the keying or switching signals is independent of the luminance, an intense, uniform background illumination without shadows thereon is no longer required. The requirements for the illumination and the construction of the more or less uniform background against which a scene is recorded are, namely, less critical.